1. Field of the Invention
The present invention generally relates to a servo unit. More particularly, the invention relates to an electromagnetic servo unit adapted to be applied to a power steering system.
2. Description of Relevant Art
In general, those electromagnetic servo systems adapted to be applied to a power steering system of a vehicle are constituted with an input shaft adapted to be operatively connected to a steering wheel of the vehicle, an output shaft adapted to be operatively connected to a steering gearbox for a steerable road wheel of the vehicle, an elastic member interposed between the input shaft and the output shaft to interconnect them with each other in a torque-transmittable manner, a torque detector for detecting steering torque applied to the input shaft, and an electric motor for providing the output shaft with assistant torque in accordance with a detection signal from the torque detector.
As an example thereof, there is disclosed an electromagnetic servo system in U.S. Pat. No. 4,448,275 (Kitagawa et al, May 15, 1985).
In the electromagnetic servo system according to the aforesaid United States Patent, a torsion spring is employed as the elastic member interposed between the input shaft and the output shaft to couple them together. A torque sensor arranged on the input and output shafts is adapted to detect the steering torque applied to the input shaft, in the form of a relative angular displacement between the shafts, as it is measured in the circumferential direction of the shafts. The electric motor for providing the output shaft with the assistant torque is disposed independently of the shafts.
In a power steering system employing an electromagnetic servo system having a torsion spring interposed between an input shaft and an output shaft as in the servo system according to the aforesaid United States Patent, it is hardly possible to achieve a steering feeling like that in a steering system of a manual type in which no power assist device is provided or in other words the input and output shafts are constituted integral with each other.
Such a shortcoming is likewise found also in a power steering system employing an electromagnetic servo system using, in place of the torsion spring, an elastic member of a different kind.
With the foregoing points in mind and with a view that, in a power steering system of a type like above, it will constitute an advantage to provide a steering feeling of a manual type steering system under a certain condition, exemplarily when the vehicle speed is relatively high, the present invention is achieved to substantiate an improvement, particularly for a power steering system to which an electromagnetic servo unit of such a type as aforementioned is applied.